The present disclosure is directed generally to frequency synthesizers.
Frequency synthesizers are widely used in modern radio communication systems. Such devices typically make use of a single quartz-controlled (i.e., crystal) reference oscillator combined with a phase-locked loop (PLL) to provide a multitude of output frequencies traceable to the highly stable reference from the oscillator.
Some frequency synthesizers employ multiple loop fine step frequency synthesis using a mix and divide technique. Other, single loop frequency synthesizers, employ digital phase detectors and integer or fractional frequency dividers. Single loop frequency synthesizers may employ a direct digital synthesizer (DDS) in the loop technique together with an analog phase detector and an auxiliary digital phase frequency detector for acquisition. Other implementations include frequency synthesizers for point to point radio supporting high data rates using complex modulation formats using carriers in the millimeter wave frequency range. These frequency synthesizers are required to perform broadband tuning, low phase noise and high frequency stability.
Shortcomings of conventional frequency synthesizers include phase noise, especially in the microwave frequency band, and tuning bandwidth limitations due to down-converting.
FIG. 1 is block diagram of a conventional frequency synthesizer 100. The frequency synthesizer 100 employs a divide by eight frequency divider 102 and a down-converting mixer 104. The divide by eight frequency divider 102 and the down-converting mixer 104 converts a signal 106 from a voltage controlled oscillator 108 (VCO) to an intermediate frequency (IF) signal 110 centered on 250 MHz for phase comparison with a variable frequency reference signal 112 generated by a DDS 114 section of a frequency synthesizer integrated circuit 116 such as an AD9858 frequency synthesizer integrated circuit available from Analog Devices, Inc. An on-chip digital phase/frequency detector 118 is employed for acquisition and an auxiliary analog phase detector 120 is employed to improve phase noise performance.
The performance of the conventional frequency synthesizer 100, however, is limited as follows. First, the phase comparison is performed using the variable frequency reference signal 112 generated directly by the DDS 114 clocked at 1 GHz. The spurious free dynamic range (SFDR) of the DDS 114 is approximately −50 dBc when it is used to generate a signal at a large fraction of the clock frequency, approximately one quarter in this case. This gives rise to troublesome spurious output signals. In telecommunications dBc indicates the relative dB levels of noise or sideband peak power, compared to the carrier power.
Second, in addition to the IF signal 110, the down-converting mixer 104 produces IF signals at frequencies IF=FSIG+FLO (the lower sideband at 250 MHz is used when Fout=10 GHz). The mixer 104 also produces unwanted IF outputs one of which is at a frequency given by: FSPUR=3FLO−2FSIG.
A first unwanted IF signal at 250 MHz will be generated when the output 122 of the synthesizer 100 is at 11 GHz because:
                                          3            ×            1000                    -                      2            ×                          (                              11000                8                            )                                      =        250                            (        1        )            
A second unwanted IF signal will be generated when FLO−FSIG=250 MHz. This will occur when the output 122 of the synthesizer 100 is at 6 GHz because:
                              1000          -                      (                          6000              8                        )                          =        250                            (        2        )            
Accordingly, the frequency synthesizer 100 may be operated only over the frequency range of:11 GHz>FOUT>6 GHz  (3)
Accordingly, there exists a need for a frequency synthesizer operable at high frequencies (such as microwave frequencies) with improved phase noise characteristics capable of being tuned over an extended bandwidth.